The　welding　joint　is　the　weak　link　of　the　high-density　polyethylene　pipeline.　The　continuous　development　of　ultrasonic　phased　array　technology　is　expected　to　replace　traditional　non-destructive　testing　methods.　In　this　paper,　ultrasonic　phased　array　technology　is　used　to　detect　the　defects　of　butt　fusion　welding.　The　results　show　that　the　ultrasonic　phased　array　can　distinguish　between　metallic　and　non-metallic　inclusions,　and　the　inclusion　size　can　be　displayed.　Besides,　insufficient　fusion　defects　have　no　obvious　defect　features　in　the　sector　scan　results.　The　types　of　insufficient　fusion　defects　can　be　judged　from　the　end-face　line　scan　results.　©　Published　under　licence　by　IOP　Publishing　Ltd.